1. Field of the Invention
The present invention relates to a method of forming multiple oxide layers with different thicknesses simultaneously, and more specifically, to a method of forming multiple oxide layers with different thicknesses in a linear nitrogen doping process.
2. Description of the Prior Art
As the integration density of the chip increases and system-on-chip develops, multiple oxide layers with different thicknesses have become a critical element in integrated circuits so as to give a semiconductor device with multiple operating voltages. For a flash memory, the operating voltages of a memory cell and a peripheral circuit are 3.3 volts and 5 volts, respectively. Thus, both the channel length of the gate and the thickness of the gate oxide of the metal-oxide semiconductor (MOS) transistor in the peripheral circuit need to be greater than those of the MOS transistor in the memory cell so as to prevent electrical breakdown caused by a high voltage. Besides, multiple oxide layers with different thicknesses are normally needed in a read only memory (ROM).
There are multiple methods of forming oxide layers with different thicknesses. In 1993 Nakata et al. (U.S. Pat. No. 5,254,489) disclosed a method of forming two oxide layers with different thicknesses. Please refer to FIG. 1A to FIG. 1D of cross-section views of forming two oxide layers with different thicknesses disclosed by Nakata et al. In U.S. Pat. No. 5,254,489. As shown in FIG. 1A, a semiconductor substrate 1 comprises two areas, portions of the semiconductor substrate 1 in each area having a first oxide layer 3 atop, isolated by a dielectric layer 2. The first oxide layer 3 is normally formed by performing a dry oxidation process with pure oxygen supplied at a temperature of between 800 and 1150xc2x0 C.
As shown in FIG. 1B, a nitridation process, with nitrogen supplied at a temperature ranging from 1000 to 1200xc2x0 C. or with ammonia gas supplied at a temperature ranging from 900xc2x0 C. to 1150xc2x0 C., is then performed on the semiconductor substrate 1 to form a nitrided oxide layer 6. As shown in FIG. 1C, a photoresist layer 4 is employed to cover portions of the nitrided oxide layer 6. A hydrofluoric acid (HF) is then employed to remove portions of the nitrifled oxide layer 6 not covered by the photoresist layer 4 so as to expose portions of the surface of the semiconductor substrate 1. Finally, as shown in FIG. 1D, an oxidation process is performed to form an oxide layer 5 with a thickness greater than a thickness of the nitrided oxide layer 6 on the exposed surface of the semiconductor substrate 1 at the end of the method.
However, the photoresist layer 4 employed In the method of forming two oxide layers with different thicknesses disclosed by Nakata et al. can easily become contaminated leading to unstable quality of the oxide layers. In addition, the quality of the nitrided oxide layer 6 is worse than that of a pure silicon oxide layers. Besides, portions of the remaining nitrided oxide layer 6 covered by a mask layer during the formation of the oxide layer 5 are lightly oxidized during the oxidation process so as to lead to a defective thickness of the nitrided oxide layer 6. Most importantly the manufacturing processes, including two lithography processes, are complicated and not practical for manufacturing multiple gate oxide layers with more than three different thicknesses.
It is therefore a primary object of the present invention to provide a method of forming multiple oxide layers with different thicknesses quickly and reliably with a high yield rate in the process.
It is another object of the present invention to provide a method of forming multiple oxide layers with different precisely controlled thicknesses by performing only one linear nitrogen doping process and one oxidation process.
According to the claimed invention, a semiconductor substrate comprising a silicon surface, further comprising at least a first region and a second region is provided in a method of forming multiple oxide layers with different thicknesses. A sacrificial oxide layer is formed on the silicon surface to cover both the first region and the second region. A mask layer is then formed on the surface of the sacrificial oxide layer. By defining and patterning the mask layer, a first opening, having a first predetermined surface area (A1), and multiple second openings, each having a second predetermined surface area (A2), are respectively formed in portions of the mask layer within the first region and portions of the mask layer within the second region to expose portions of the sacrificial oxide layer having a surface area equal to the first predetermined surface area, and portions of the sacrificial oxide layer having a surface area equal to the second predetermined surface area, respectively. A linear nitrogen doping process is performed to simultaneously implant nitrogen ions with a first predetermined concentration(C1) and nitrogen ions with a second predetermined concentration(C2) into the first region and the second region, respectively, through the first opening and the second opening, respectively. The mask layer is then removed. Finally, the sacrificial oxide layer is removed and an oxidation process is performed to form a first silicon oxide layer, having a first predetermined thickness, and a second silicon oxide layer having a second predetermined thickness, in the first and second regions, respectively.
A ratio of the first predetermined surface area to the second predetermined surface area is defined as a constant k, obeying the following equation:
A1/A2=kxc3x97(C1/C2)
In the preferred embodiment of the present invention, the first predetermined surface area is greater than the second predetermined surface area, the first predetermined concentration is greater than the second predetermined concentration and the first predetermined thickness is less than the second predetermined thickness.
It is an advantage of the present invention against the prior art that only one nitrogen ion implantation process and one thermal oxidation process are needed to simultaneously form multiple gate oxide layers with different thicknesses. In addition, the different thicknesses of the multiple gate oxide layers can be precisely controlled. Most importantly, the quality of the gate oxide layer formed in the present invention Is better than that formed in the prior art, and the manufacturing processes are simplified. Consequently, the yield rate is efficiently improved and the manufacturing cost is significantly reduced.